如何通过PHP Socket SMTP握手验证邮箱有效性？代码误判问题

我希望通过PHP Socket与SMTP服务器握手，验证收件人邮箱地址是否有效，但当前代码总是将真实存在的邮箱判定为无效。

以下是我用于验证的代码：

$smtpHost = 'mail.sender.com'; // Replace with your SMTP server hostname
$smtpPort = 587; // Use the appropriate port for your SMTP server
$localDomain = 'sender.com'; // Replace with the name of your server
$recipientEmail = 'noreply@receiver.com'; // Replace with the recipient's email
$username = 'noreply@sender.com';
$password = '**********';
list(,$domain) = explode("@",$recipientEmail);

// Open a socket connection to the SMTP server
$socket = fsockopen($smtpHost, $smtpPort, $errno, $errstr, 30);

if (!$socket) {
    echo "Failed to open socket: $errstr ($errno)";
} else {
    
    function readResponse($socket) {
        $response = '';
        do {
            $line = fgets($socket, 512);
            $response .= $line;
        } while (preg_match('/^\d{3}-/', $line));

        echo $response;
        return $response;
    }

    // Read the initial response from the server
    $response = readResponse($socket);

    // Send the EHLO (or HELO) command to the SMTP server
    fputs($socket, "EHLO $localDomain\r\n"); // Use EHLO or HELO as needed
    $response = readResponse($socket);

    // Check if the server responds with a 250 code indicating a successful greeting
    if (preg_match('/^250/', $response)) {
        // Send the AUTH command with your SMTP credentials (username and password)
        $response = readResponse($socket);
        fputs($socket, "AUTH LOGIN\r\n");
        $response = readResponse($socket);

        if (preg_match('/^334/', $response)) {
            // Send the base64-encoded username
            fputs($socket, base64_encode($username) . "\r\n");
            $response = readResponse($socket);

            if (preg_match('/^334/', $response)) {
                // Send the base64-encoded password
                fputs($socket, base64_encode($password) . "\r\n");
                $response = readResponse($socket);

                // Check if authentication was successful (usually 235 for success)
                if (preg_match('/^235/', $response)) {
                    
                    // Send the sender's email address
                    fputs($socket, "MAIL FROM:<$username>\r\n");
                    $response = readResponse($socket);
                    
                    // Now you can send the RCPT TO command
                    fputs($socket, "RCPT TO:<$recipientEmail>\r\n");
                    $response = readResponse($socket);
                    
                    // Send VRFY command
                    fputs($socket, "VRFY $recipientEmail\r\n");
                    $response = readResponse($socket);
                    
                    // Check the response
                    if (preg_match('/^250/', $response)) {
                        echo "The email address '$recipientEmail' exists on the server.\r\n";
                    } else {
                        echo "The email address '$recipientEmail' does not exist or the server doesn't support VRFY.\r\n";
                    }

                    
                    // Close the connection
                    fputs($socket, "QUIT\r\n");
                    $response = readResponse($socket);
                    
                    
                } else {
                    echo "SMTP authentication failed.\r\n";
                }
            } else {
                echo "Failed to send the password.\r\n";
            }
        } else {
            echo "Failed to send the username.\r\n";
        }
    } else {
        echo "Failed to greet the SMTP server.\r\n";
    }

    // Close the socket connection when done
    fclose($socket);
}

代码运行后，响应提示“该邮箱不存在或服务器不支持VRFY”，但实际该邮箱是真实存在的，请求协助解决代码的误判问题。

内容的提问来源于stack exchange，提问作者Waseem Jutt